Ceramic bodies and their production



Feb. 9, 1965 F. A. HUMMEL CERAMIC BODIES AND THEIR PRODUCTION 2 Sheets-Sheet 1 Filed March 20, 1965 J I N N D w m l T T U ll|lll lllll I: E llllllllrllll m A m mw L M a B Lm sm l mwmm WSP S E S u w M I EDT I I L L.. 00 "W58 0 2 0 O 0 0 O O l. 6 O O 0 O m l W. W m m 8 8o. uma mmmxmh IMOLE MsaSiQ SYSTEM ZmgSiOvMezSiOa FLOYD A. HUMMEL INVENTOR.

BY Wm ATTORNEY Feb. 9, 19 65 F. A. HUMMEL J CERAMIC BODIES AND THEIR PRODUCTION Eiled March 20, 1963 2 Sheets-Sheet 2 BATCH MATERIALS PROVIDING SE02, ZNO AND M00 IN PROPORTIONS THEORETICALLY REQUIRED TO FORM SOLID SOLUTION OF 05-42 MOLE PERCENT MGZSIOA IN ZNQSIOAI MIXIN 6 AND COMMINUTING CALCINING AT LEAST 50% OF MIXED BATCH MATERIAL COMMINUTING REMIXING ENTIRE BATCH MATERIAL WITH A COMBUSTIBLE BINDER FORMING MIXTURE INTO BODY OF DESIRED SHAPE FIRING BODY TO REACT AND SINTER IXTURE COOLING THE FIRED BODY FLOYD A. HUMMEL INVENTOR.

IITWKNEY rates ate r York Filed Mar. 20, 1963, SenNo. 266,579 12 Claims. (Cl. 10639) This invention relates to mechanically strong, refractory, low porosity, low expansion, single phase ceramic bodies and the method for their production. More particularly, this invention relates to the production of ceramic bodies with the foregoing characteristics and consisting essentially of a single phase Willemite solid solutiOH Of in ZH SlO During the past decade, considerable need has developed for strong, refractory, low expansion, single phase ceramic bodies. Applications such as vacuum tight envelopes, combustion chambers, firing trays and aircraft parts need materials with the properties mentioned above. Often, difficulties of fabrication limit the use of these materials. 7

it is an object of this invention to provide mechanically strong, refractory, low porosity, low expansion, substantially Wholly single phase Willemite solid solution ceramic bodies.

It is another obect of this invention to provide a relatively simple and easy process of fabricating the aforementioned bodies.

It is still another object of this invention to provide ceramic bodies of the aforementioned type that are substantially non-porous.

Other objects and advantages of the present invention will become apparent, to those skilled in the art, from the following detailed description in conjunction with the attached drawing.

FIGURE 1 is a partial phase diagram of the system Zn SiO Mg SiO FIGURE 2 is a schematic flow diagram of the process according to the present invention.

Referring now to FIGURE 1, it can be seen that single phase willemite solid solutions can be formed when magnesium orthosilicate (Mg SiO does not exceed about 44 mole percent. However, as a practical matter, it is diflicult to produce single phase bodies according to the process of the invention when the magnesium orthosilicate content closely approaches the 44 mole percent limit; therefore, it is generally desirable to keep the maximum limit for magnesium orthosilicate down to about 42 mole percent.

Substantially non-porous bodies can be produced with the magnesium orthosilicate content maintained in the range of about 30 to 42 mole percent. When the mag nesium orthosilicate content is lower than about 30 mole percent, small amounts of porosity are found in the bodies and the amount of porosity tends to increase as the magnesium orthosilicate content is decreased. Where extremely low porosity is not essential, the magnesium orthosilicate can be as low as about 0.5 mole percent with the balance being substantially all zinc orthosilicate (Zn Si and still provide a strong, refractory, low expansion, single phase body.

The magnesium orthosilicate content in bodies of the present invention provide an important inhibiting effect on the volatilization of ZnO from the zinc orthosilicate in the body during firing and during later elevated temperature service. Bodies made essentially wholly of Zinc orthosilicate lose ZnO by volatilization with the consequent formation of uncombined silica in the bodies. This fr e silica greatly tends to form a glassy phase in the bodies thereby adversely affecting the desired properties, e.g., refractoriness and hot strength are substantially redddtldlm Patented Feb. 9, 19%5 ducecl. The presence of magnesium orthosilicate in the bodies of the present invention significantly inhibits the volatilization of ZnO therefrom and the adverse effects resultingfrom such volatilization.

The method of producing bodies according to this invention utilizes common, readily available raw materials that are reacted at elevated temperatures to form essentially single phase willemite solid solutions. Thus, commercially available materials that will provide the basic oxidic constituents of SiO ZnO and MgC, Without produ'cing any substantial impurity content, can be used. Commonly known potters flint has been found suitable for providing the silica content. The zinc oxide content has been provided by U.S.P. zinc oxide and the magnesia content by GP. magnesium carbonate.

Referring now to FIGURE 2, the process of the present invention comprises mixing oxidic batch material consisting essentially of Si0 ZnO and MgO in proportions theoretically required to form a substantially wholly single phase solid solution of about 0.5 to 42 mole percent of Mg SiO in Zn SiO upon substantially complete reaction of all the batch material. Preferably, the batch material is mixed in a ball mill to insure a homogeneous and finely divided mixture for proper and complete reaction in the subsequent steps.

It has been found that bodies having the previously stated desired properties cannot be successfully made by forming bodies wholly from unreacted batch material and then firing to completely react the constituents of the batch. The problem arises from the fact that as zinc orthosilicate is formed from ZnO and SiO a very considerable volume expansion takes place. The entire reaction of all the batch material is done in one step after the green ceramic body has been formed. This volume expansion accompanying the formation of zinc orthosilicate causes structural disruptions, cracking, lower density, higher porosity and low strength. In order to overcome this problem, at least about 50%, by weight, of the batch mixture has to be at least partially pre-reacted by calcining at a temperature at least sufiicient to form the zinc orthosilicate. Preferably the calcining temperature is not high enough to cause any substantial sintering. A temperature range of about between 1000 and 1300 C. is suitable. When desired or economically practical, the entire batch mixture can be pre-reacted; however, it is preferred to pre-react only about two-thirds of the batch mixture for optimum results.

After the calcining step, the pro-reacted material is comminuted to a finely divided state comparable to that of the unreacted batch mixture, which should be about minus 44 mesh (Tyler). Then the entire batch mixture is remixed together with a combustible binder and formed into a green ceramic body of desired shape by any suitable means, e.g., by pressing, extrusion, etc. Compacting shaped bodies by pressing the binder-containing batch material has been found suitable and quite economical for most shapes and sizes. Batch mixtures for pressing should contain a combustible binder content of at least about 3%, but not in excess of about 15%, by weight of the whole remixed batch material. Any suitable material commonly used in the ceramic art as a combustible binder can be used, e.g., a solution of Carbowax #4000 or #6000 which are waxy polyethylene glycols having molecular Weights of about 4000 and 6000, respectively, sold by the Union Carbide Corporation and are water soluble as well as soluble in a wide range of organic solvents.

The green ceramic body is fired to a temperature of not less than about 1300 C. but sufficient to substantially completely react all the batch ingredients to form an essentially single phase solid solution of Mg SiO in Zn SiO without causing any melting of the batch ma- 'forste-rite solid solution upon cooling to room temperaterial and for a time of at least about one-half hour but of the following temperatures: 25 C., 200 0, 400 C., sufficient to provide a mechanically strong, low porosity 600 C., 800 C., and 1000 C. The average modulus body. It will be apparent from FIGURE 1 that, for of rupture over the temperature range of 25-600 C. bodies having an analytical composition that will pro- Was substantially constant at a median value of about vide more than about 37 mole percent Mg SiO the firing 5 7,370 p.s.i. A-t 800 C., average modulus of rupture temperature will also have to be no lower than line A. was found to be 7,890 p.s.i. and then it dropped off to Of course, firing temperatures somewhat higher than line 6,300 p.s.i. at 1000 C. Youngs modulus values were B of FIGURE 1 will lead to detrimental incipient melting. determined on each of three bars for each of the same While the phase diagram shown in FIGURE 1 appears to temperatures at which modulus of rupture values were indicate that many compositions of this invention will determined. Over the temperature range of 25 600 form a tWo phase mixture of willemite solid solution and C., the average Youngs modulus was about ll.2i0.5 10- p.s.i. A high average value of about 12.6 l0- p.s.i. was obtained at 800 C. and at 1000 C. the average value was slightly less than 6 10- p.s.i.

Thermal expansion data was obtained, by known conventional procedures, for the above example over the temperature range of 25-1000 C. and the coefiicient for this temperature range was determined to be 32 10- C.

As further illustrations of the invention, other bar specimens were prepared from batch material mixtures made up as follows, in mole percent, from the same raw materials as in the previous example:

ture, it has been found that even with fairly slow cooling rates (as by cooling down with the furnace) no second phase forsterite solid solution is found to appear in bodies made according to this invention. Apparently the transformation is extremely sluggish.

It should be noted that, in proportioning the batch ingredients and in firing the green ceramic bodies, care should be taken to avoid free or uncombined silica in the final product because it produces substantially the same detriment as in the Wholly zinc orthosilicate bodies previously mentioned. Small amounts of impurities, or second phase, such as magnesium metasilicate (MgSiO unreacted magnesia and/or zinc oxide, can be tolerated Batch without too much adverse effect, but generally, the im- SiOr ZnO MgO purities should not exceed 2 mole percent. In many 33.33 56.67 10.00 cases, small amounts of unreacted magnesia and/or zinc 33-33 50-00 15157 A4. 33. 33 43. as 23. 3a oxide are unavo1dable in order to assure the absence of free or unreacted silica.

The following example is given to better illustrate the invention.

A batch material mixture was made up as follows:

The bars were prepared in the same manner as the previous example except that they were fired at 1300 C. for 3.5 hours. The approximate composition (in mole percent) of the single phase solid solution fired bars and 35 apparent porosity were as follows:

Oxide Source Wt. Mole percent percent Bar Mg2SiO4 ZnzSlO; Percent Porosity SiOz Potter's flint 30. 94 33. 33 ZnO U.S.P. zinc oxide 54. 52 MgO 0.1. basic MgCO; 14. 54 15 85 15. 7 25 75 6.9 35 65 0. 0

A three kilogram quantity of the above batch was milled as a thick aqueous slip for 24 hours in a ball mill. For om arison purposes, a bar made wholly of potters After drying at C: One-half 0f the material Was fi flint and U.S.P. zinc oxide in proportions to theoretically cined at 1260 C. for 2 hours and ground to minus 60 yield a single phase of only Zn SiO was made in the mesh y Remixtfd batches Were made P in 100 same manner as the latter three of the foregoing exgram samples in which the calcined material constituted 3mp1es Apparent o osit f thi ba was 25.9% and it two-thirds of the ceramic material and the unca ci had considerably lower strength than bodies of this invenmaterial constituted the remaining one-third portion. Ten 11 grams of a binder solution consisting of y Weight) Thus, it can be seen that, by the present invention, 05% melhylceuulose -z dimethyl ether 0f 0611111086), there is provided novel low expansion refractory ceramic 33.1% CurbOW'dX 000 and 6 Water w s thOrOughbodies having good mechanical strength and low porosity. 1y mixed into each of the 100 gram samples with a mOr- Over the temperature range of 25 to 1000" C., bodies of tar and pestle. This mixture was then nodulized by passthe present invention are able to withstand rupture loads g it through a 20 mesh W The nodulized of at least 4,000 p.s.i. with an apparent porosity of no Samples were then Pressed into bars pp y more than 20% and at least 6,000 p.s.i. with apparaent cm. x 1 cm. x 8 cm.) in a steel mold under a pressure porosity closely approaching zero percent. of 600 p s.i. After the green ars e removed from Although the present invention has been described with the mold. hey were heated at a rate of 90 C. per hour respect to specific details of certain embodiments thereof, to 1425 3-, held for 2 hours at 1425 and h it is not intended that such details be limitations upon furnace cooled. The cooled bars exhibited a single phase the scope of the invention except insofar as set forth in solid solution of about 35 mole percent of Mg SiO in the cl ims. 7 65 mole percent of Zn SiO Apparent porosity measure- What i claimed is; m s Of the bars e according to A.S.T.M. Designa- 1. As an article of manufacture, a mechanically strong, tion C373-55T, 1955 Book of A.S.T.M. Standards, Part 55 low porosity, low expansion ceramic body consisting es- 3, pp. 81840) showed values of less than 0.1%. This sentially all of a single phase solid solution of Mg SiO non-porous characteristic was further confirmed by elecin Zn SiO wherein the Mg SiO is analytically present in tron microscopy and by water absorption test values of an amount of about 0.5 to 42 mole percent and the 0.0%. remainder is substantially all Zn SiO Modulus of rupture (sometimes called transverse 2. A ceramic body according to claim 1 having a modstrength) and Youngs modulus values were determined ulus of rupture of at least about 4,000 p.s.i. at temperain the manner previously described in a paper by Bush tures in the range of 25 to 1000 C. and an apparent and Hummel: Journal of the American Ceramic Society, porosity not in excess of about 20%. vol, 41 (1958), No. 6, pages 189-195. Modulus of rup- 3. As an article of manufacture, a mechanically strong, ture values were determined on each of ten bars for each 7 substantially non-porous, low expansion ceramic body consisting essentially all of a single phase solid solution of Mg SiO in Zn SiO wherein the Mg SiO is analytically present in an amount of about 30 to 42 mole percent and the remainder is substantially all Zn SiO 4. A ceramic body according to claim 3 having a modulus of rupture of at least about 6,000 p.s.i. at temperatures in the range of 25 to 1000 C. and an apparent porosity of substantially zero percent.

5. The method of forming a mechanically strong, low porosity, low expansion, substantially single phase ceramic body comprising:

(a) mixing finely divided oxidic batch material consisting essentially of SiO ZnO and MgO in proportions theoretically required to form a substantially wholly single phase solid solution of about 0.5 to 42 mole percent Mg SiO in Zn SiO upon substantially complete reaction of all the batch material,

(b) calcining at least about 50%, by weight, of the batch mixture material at a temperature at least sufficient to partially pre-react the mixture to form the ZI1 SiO (c) comminuting the calcined batch mixture material,

(01) remixing the entire batch mixture material together with a combustible binder,

(e) forming the binder-containing batch mixture material into a green ceramic body of desired shape,

(f) firing the green ceramic body to a temperature of not less than about 1300 C. but sufiicient to substantially completely react all the batch material to form the said single phase solid solution without causing any melting of the batch material and for a time of at least about one-half hour but sufiicient to provide a mechanically strong, low porosity body.

6. The method according to claim 5 wherein approxi- 35 mately two-thirds of said batch mixture material is calcined.

7. The method according to claim 5 wherein said calcining is done in the temperature range of about 1000 to 1300 C.

8. The method according to claim 5 wherein the firing is done for a time suificient to provide the said body with a modulus of rupture over the temperature range of 25 to 1000 C. of at least about 4,000 p.s.i. and with an apparent porosity not in excess of about 20%.

9. The method according to claim 5 wherein said finely divided 'oxidic batch material is mixed in proportions theoretically required to form a substantially wholly single phase solid solution of about 30 to 42 mole percent Mg SiO in Zn SiO upon substantially complete reaction of all the batch material.

10. The method according to claim 9 wherein approximately two-thirds of said batch mixture material is calcined.

11. The method according to claim 9 wherein said calcining is done in the temperature range of about 1000 to 1300 C.

12. The method according to claim 9 wherein the firing is done for a time sufficient to provide the said body with a modulus of rupture over the temperature range of 25 to 1000" C. of at least about 6,000 psi. and with an apparent porosity of substantially zero percent.

References Cited in the file of this patent UNITED STATES PATENTS 2,139,686 Lederle et a1 Dec. 13, 1938 FOREIGN PATENTS 384,473 Great Britain Dec. 8, 1932 

1. AS AN ARTICLE OF MANUFACTURE, A MECHANICALLY STRONG, LOW POROSITY, LOW EXPANSION CERAMIC BODY CONSISTING ESSENTIALLY ALL OF A SINGLE PHASE SOLID SOLUTION OF MG2SIO4 IN ZN2SIO4 WHEREIN THE MG2SIO4 IS ANALYTICALLY PRESENT IN AN AMOUNT OF ABOUT 0.5 TO 42 MOLE PERCENT AND THE REMAIDER IS SUBSTANTIALLY ALL ZN2SIO4. 